Resonant cavity filters are used in many high frequency (RF and microwave) electronic applications. For example, in cellular telephone communications, users within each operating cell are assigned a unique operating frequency within the frequency band designated for cellular communications. Therefore, each time a cellular user places or receives a call, that call will be assigned to one of hundreds of allocated frequencies. The transmitter channel in the cell repeater station that is relaying the telephone call must be tuned to the specific frequency of the call. A typical cellular communications frequency band spans 869 MHz-894 MHz, with channel frequencies spaced 630 kHz apart (Advanced Mobile Phone Service (AMPS) frequency standard). The cellular telephone service provider will assign particular channel frequencies to different cell sites within its service area. For example, a typical cell site may have 24 channel frequencies assigned to it. Each of these channels has a repeater transmitter that operates at the channel frequency.
Typically, each channel in the cell station has a dielectric resonant filter at the transmitter's RF output that must be tuned to the channel frequency. This narrow bandpass filter ensures that only the frequency assigned to that channel is transmitted. FIG. 1 schematically illustrates such a prior art resonant cavity filter, indicated generally at 10. Filter 10 comprises a cavity enclosure 12 substantially surrounding a dielectric resonator 14. Dielectric resonator 14 is typically composed of barium tetratianate (BaT.sub.14 O.sub.9). An input signal from the channel transmitter is received on the input to the cavity via conductor 16, terminal 17 and tuning loop 18. It is the function of the resonant cavity filter 10 to form a bandpass which attenuates all signals except the assigned channel frequency. The resonant frequency of the filter 10 is changed by increasing or decreasing the effective volume of cavity 12. This volume change is affected by varying the position of a tuning plate 20. Tuning plate 20 is moved coaxially within the cavity 12 by means of an adjustment screw 22 coupled to a tuning shaft 24, which is in turn coupled to tuning plate 20. Rotation of the adjustment screw 22 moves tuning shaft 24 into or out of the cavity 12, depending upon the direction of rotation of the adjustment screw 22. Dielectric resonator 14 is excited by the RF input signal emanating from tuning loop 18 and this causes resonator 14 to vibrate. However, resonator 14 will vibrate substantially only at a resonant frequency determined by the physical dimensions of the cavity 12. The resulting filtered output is coupled by tuning loop 28 and therefore consists mainly of the resonant frequency component of the RF input signal. Tuning loop 28 is coupled to terminal 29 and conductor 30.
There are certain situations where the cellular service provider would like to reallocate channel frequencies temporarily from one cell to another. For example, when a large number of people congregate in one place, such as at a sporting event, a very large number of cellular users are placed into one cell site. The number of channel frequencies assigned to that cell site may very well be inadequate to handle the increased demand for channels. In this situation, the cellular service provider will want to temporarily reassign channel frequencies from other cell sites. Present technology enables the channel transmitters to be tuned to particular frequencies under remote control (such as through a telephone line), however, the resonant cavity filters 10 of each new channel must be manually tuned to the new frequency. This manual operation is both time consuming and expensive. During manual tuning, the tuning plate 20 must be moved until the effective volume of the cavity 12 is such that the dielectric resonator 14 resonates at the channel frequency and therefore only the assigned frequency will pass through filter 10. To do this, the reflected power at the input terminal 17 is measured. When the resonant cavity filter 10 is properly tuned, the reflected power at input terminal 17 will be at a minimum. Therefore, the tuning sequence begins with the operator rotating the adjustment screw 22 to move the tuning plate 20 in a first direction. If the reflected power at input terminal 17 increases, the operator moves the tuning plate 20 in the opposite direction. If, on the other hand, the reflected power at input terminal 17 decreases, the operator continues to rotate the adjustment screw 22 to move the tuning plate 20 in the same direction until the reflected power ceases to decrease. At this point, the reflected power is at a minimum and the resonant cavity 10 is therefore tuned to the channel frequency (the frequency of the channel transmitter).
The prior art resonant cavity filter 10 of FIG. 1 has a major problem. A human operator must adjust the resonant frequency of each newly assigned channel at the cell site even though the frequency of the channel transmitter can be changed from a remote location. It can take quite awhile for the operator to perform this operation because he must move the tuning plate 20 back and forth in small steps over a potentially great distance in order to discover the minimum reflected energy. If the newly assigned channel is using a frequency much higher or much lower than the previously used frequency, the tuning plate must be moved relatively far within the cavity 12 while searching for the minimum frequency. This process involves moving the tuning plate 20 a predetermined step size, measuring the reflected power, and determining if the newly measured reflected power is greater than or less than the previously measured reflected power for every step increment. The smaller the predetermined step size, the more accurately tunable is the filter 10. Therefore, a precise resonant cavity filter 10 can take quite a while to determine the optimum tuning position of the tuning plate 20.
Accordingly, a self-tuning resonant cavity filter which overcomes any or all of these problems is highly desirable. The present invention is directed toward meeting these needs.